I don't have that feeling about space, but about deep water. There could be anything down there. Sharks, jellyfish, enormous whales that could swallow me in one gulp. I would never go scuba diving in the ocean for this reason. Yet somehow I find it fascinating at the same time.

Sorry that I don't have any constructive advice. If it's not debilitating, maybe try to appreciate the fear -- after all, it's part of what makes us human? If it is debilitating, probably best to consult a professional.